1. Field of the Invention
The present invention relates to a water-absorbent and water-retention material comprising a water-absorbent resin which is excellent in stability and has salt resistance and relates to the method for producing the same. More particularly, the present invention relates to a water-absorbent or water-retention material comprising a water-absorbent resin derived from unit components comprising hydroxyl group-containing water soluble mono-(meth)acrylate, (meth)acrylic acid and/or alkali metal salt thereof and crosslinking agent at a predetermined ratio as essential components. The present invention also relates to a water-absorbent or water-retention material comprising a thermally-crosslinked product of a copolymer of (meth)acrylamide and (meth)acrylic acid (alkali metal salt). Herein, the term "(meth)acryl-" denotes "acryl-" and/or "methacryl-". Moreover, at least one kind of monomer selected from the group consisting of acrylic acid, methacrylic acid, alkali metal salt of acrylic acid, and alkali metal salt of methacrylic acid will be generally named as "(meth)acrylic acid (alkali metal salt)."
2. Description of the Prior Art
Hitherto, water-absorbent resins have been widely used in hygienic materials such as sanitary materials and paper diapers, water retainers for soil and the like. Water insoluble crosslinked polymers are known as such water-absorbent resins. Examples of the above mentioned water-absorbent resins include crosslinked polyacrylic acid salts, self-crosslinking type polyacrylic acid salts, and crosslinked copolymer of starch-grafted acrylic acid salts.
Such acrylic water-absorbent resins are highly absorbent for solutions having relatively low concentration of ions, for example water or urine, but the absorbency is much less for an aqueous liquid containing a high concentration of metal ion, for example sea water or a cement slurry that contains a large amount of calcium ion. Moreover, in a case where an acrylic water-absorbent resin is used in the form of a hydrogel, under a condition in which free radicals are generated in a molecule, for example, by exposure to the light or by being left at a high temperature for a long time, a cleavage in the main chain of the polymer occurs due to the radicals. As a result, the gel elasticity deteriorates significantly. Furthermore, the acrylic resins cannot retain the form of a gel.
As a water-absorbent resin which increases the absorbency of an aqueous liquid containing metal ions, for example, sea water, cement slurry or the like, the following 1 to 3 have been suggested.
1 A crosslinked polymer of sulfonic acid monomer (salt) or a crosslinked copolymer of sulfonic acid monomer (salt) with acrylic acid (salt) (disclosed for example in Japanese Laid Open Patent Application Tokkai Sho No. 56-161412, Tokkai Sho No. 61-36309 and others). Herein, "sulfonic acid monomer (salt)" denotes sulfonic acid monomer and/or sulfonic acid salt monomer; and "acrylic acid (salt)" denotes acrylic acid and/or acrylic acid salt. Similar expressions are used hereinafter.
2 A crosslinked copolymer of acrylic acid salt and poly (vinyl alcohol) (PVA) (disclosed in Japanese Laid Open Patent Application Tokkai Sho No. 53-104691.)
3 A crosslinked copolymer of acrylamide and acrylic acid (salt) (disclosed in Japanese Laid Open Patent Application Tokkai Hei No. 4-45850.) However, in 1 above, the dissociation degree of sulfonic acid is higher than that of carboxylic acid. Therefore, the polymer of 1 above has relatively high absorbency of an aqueous liquid containing a salt of monovalent metal ion, for example sea water, but in an aqueous liquid containing polyvalent metal ion, for example in a cement slurry containing a large amount of polyvalent metal such as calcium ions, a crosslinking occurs due to the polyvalent metal, so that the absorbency deteriorates.
Moreover, in general, the polymer of 1 above copolymerizes mainly with sulfonic acid monomer. However, this sulfonic acid monomer does not polymerize sufficiently, so that the molecular weight of the polymer is not easily increased, and the gel elasticity of the obtained polymer is not excellent. Moreover, the market price of such sulfonic acid monomer is high, and thus this type of polymer is uneconomical.
In 2 above, PVA which is a copolymerization component is a hydrophilic nonionic compound. If the compolymerization ratio of PVA is increased, the absorbency is relatively increased in the presence of a great amount of a polyvalent metal salt such as calcium. However, the crosslinking copolymer of PVA-acrylic acid (salt) is generally produced by copolymerizing acrylate, vinyl acetate and hydrophobic crosslinking agent and then saponifying with alcoholic alkali. Therefore, the molecular weight of the crosslinking copolymer cannot easily be increased and the gel strength of the polymer in a solution of metal salt is weak as compared with a polymer produced by the general aqueous solution polymerization.
Moreover, during saponification, excess alkali or by-produced sodium acetate or methanol must be removed by washing, thus making the process complicated.
In 3 above, since acrylamide is a hydrophilic nonionic compound and the polymerization property of monomer is excellent, the absorbency of an aqueous salt solution containing a great amount of polyvalent metal salt and the gel strength are relatively good as compared with the materials described in the 1 and 2 above. However, polymerization is conducted in the presence of a crosslinking agent, so that a crosslinking structure is homogeneous. Therefore, if a crosslinking degree is low, the gel strength of the obtained polymer deteriorates. On the other hand, if a crosslinking degree is high, the absorbency of the obtained polymer deteriorates. Consequently, it is difficult to attain both sufficient absorbency and sufficient gel strength.
The present invention seeks to solve the above mentioned problems in the 1-3 above and is directed to a water-absorbent resin derived from monomer unit components comprising, at a predetermined ratio, hydroxyl group-containing water soluble mono-(meth)acrylate and (meth)acrylic acid (alkali metal salt). The above mentioned water-absorbent resin of the present invention has: high absorbency of aqueous liquid having a great amount of polyvalent metal, for example a cement slurry; high gel elasticity of a polymer; and excellent gel stability when exposed to the light. Moreover, in a case where a water-absorbent resin of the present invention has a structure having a resistance to alkaline hydrolysis, the gel stability is excellent even if exposed to high temperature under alkaline conditions; and a thermally crosslinked product of a copolymer comprising, at the predetermined ratio, (meth)acrylamide and (meth)acrylic acid (alkali metal salt) has an extremely low crosslinking degree at the inner part of a resin particle and a high crosslinking degree in the outer surface part of a resin particle, and so that the thermally crosslinking product has high absorbency of aqueous liquids containing a large amount of polyvalent metal such as a cement slurry or the like.